Allow extending the same command groups by adding different extensions

[pPrecel]

Description

Some modules may want to extend the same command group (for example, the app command group). We should add support for operations described in the example:

Extension 1 structure:

- app (no execution)
  - expose
  - deploy-demo

Extension 2 structure:

- app (no execution)
  - diagnose
  - metrics

Final kyma app --help execution:

Use this command to manage applications on the Kubernetes cluster.

Usage:
  kyma app [command]

Available Commands:
  push        # built in
  expose      # from ext1
  deploy-demo # from ext1
  diagnose    # from ext2
  metrics     # from ext2